DC brushless motors are synchronous machines having a permanently magnetized rotor free to rotate within fixed stator coils. Phased alternating currents passing through the stator coils generate a magnetic field that rotates the rotor.
The phasing of the stator current with respect to the rotor position is provided by means of one or more rotor position sensing elements such as Hall effect or photoelectric devices that, by tracking a magnet or rotating opaque vane attached to the rotor, provide a "commutation" signal indicating rotor position. The commutation signal is used to control multiple solid state devices arranged in a bridge configuration to switch on and off a DC voltage so as to produce an AC driving voltage needed to apply the proper polarity of current to stator windings. The solid state switches may form a pulse-width modulated (PWM) inverter providing arbitrarily precise approximations of the needed AC driving voltages. For DC brushless motors that have multiple stator coils, multiple AC driving waveforms of different phases may be developed by multiple PWM inverters.
It is well known to control a brushless DC motor by means of one or more feedback loops. For example, in a torque control, the amplitude of the voltages applied to the motor terminals is adjusted to provide a predetermined amount of motor current, the latter which approximates motor torque. A torque command is compared to the motor torque represented by the motor current and the difference or error between these values is used to adjust the voltage applied to the motor.
It may be necessary to rapidly stop a rotating DC brushless motor in response to an emergency stop command. Such rapid stopping requires that the kinetic energy of the rotating motor shaft be quickly dissipated. The simplest method is a mechanical brake in which friction, such as between brake pads and a rotating surface, dissipates the kinetic energy as heat.
Dynamic braking makes use of the fact that the coasting DC motor acts like an electrical generator. In dynamic braking, a resistance is shunted across the stator windings allowing the energy of the coasting rotor to be converted to electrical energy and dissipated within the resistance as heat. In contrast, it should be noted that disconnecting the brushless DC motor from the source of power, insofar as it prevents current flow, eliminating the possibility of dissipating the rotors energy electrically because energy transfer out of the motor requires current flow.
While dynamic braking is relatively simple, it requires additional switching circuits and shunting resistors. Further, the effectiveness of such dynamic braking decreases rapidly as the motor speed drops thus causing an undesirably slow decay to zero RPM.
Another type of braking known in the art uses the commutating signal to create a "reverse" current in the stator windings thus creating a counter rotating magnetic field putting a reverse torque on the rotor. Such a reverse current system requires additional rephasing circuitry and relies on the availability of the commutation signal. Yet it is possible that in an emergency stop situation that commutation information will have been lost.
Accordingly, it would be desirable to have a means of braking brushless DC motors rapidly without complex circuitry or the need for commutation information.